This invention relates to a method of measuring dimensional changes in a workpiece and, more particularly, to such a method employing an image transfer technique.
Several problems exist with current techniques for measuring for dimensional variations, strain, etc., in structural materials subjected to relatively high mechanical and/or thermal loads, such as those incorporated in furnaces, heat exchangers, etc.
One of the most popular measuring techniques of this type utilize a strain gauge which is mechanically mounted to the surface of the structure to be measured. Although reasonably accurate under certain conditions, strain gauges will undergo major changes in their electrical and mechanical properties if subjected to an elevated temperature environment, which frequently introduces errors which can be in the order of .+-. 100 percent. Therefore, their practical use is limited to tests in which the workpiece is not subjected to extremely high temperatures. Also, strain gauges can only be used to measure relatively small strains which are generally less than 0.001 inches per inch, and, in general, can only be affixed to a flat surface which does not warp significantly during the test. The use of strain gauges also requires time-consuming and expensive bonding methods to affix the gauges to the surface of the structure to be tested, as well as the use of expensive electronic signal conditioning and readout equipment.
Other types of dimensional variation measurment methods include optical techniques utilizing photographic etching, holograph methods, etc. However, these techniques require extensive setup procedures and precise positioning of the optical equipment directly on the job sight. In addition to being time-consuming, these physical limitations render the optical methods impractical for many applications.
Other dimensional variation measurement techniques utilize various mechanical devices, such as vernier calipers, special micrometer fixtures, etc., which have as a major disadvantage the limitation that they cannot be used at elevated temperatures. Also, these devices cannot be used to measure variations in curved surfaced structures, and have inherent limitations on their accuracy.